Crankshafts for large internal combustion engines are generally forged as a single piece, and are attached to several cylinders. Motorcycle engines, scooter engines, and all other small internal combustion engines, generally have only one or two cylinders, and are designed to operate at high revolutions per minute during operation. These small engine crankshafts are often assembled from a set of forged and precisely machined parts, rather than forged as a single piece.
A crankshaft, that is assembled, has several common pieces for each engine cylinder. These pans .comprise a one-piece connecting rod, a big end pin, and at least two counter weight discs. End journals are attached to the counter weight discs, with the end journals being the end of the crankshaft. The end journals and counter weight discs may also be forged as a single piece. The combination of end journal and counter weight disc, whether forged as a single piece or assembled together, is termed a sub assembly. All of these pans are precisely machined before they are assembled together to form a single crankshaft. This "assembled" crankshaft poses a special problem for balancing, as opposed to a single forged piece crankshaft. An assembled crankshaft has connecting rods that are assembled together with other parts of the crankshaft during the assembly process. Forged crankshafts have two piece connecting rods, that allow them to be connected to a forged single piece crankshaft after the balancing process. With a single cylinder, or very few cylinders in the engine, the need for accurate balancing is heightened, since these smaller engines are often designed to turn at high speeds, which heighten the effects of any imbalance.
Imbalance occurs primarily due to imperfection of parts, on account of the forging and machining process. Any irregularities cause unbalance during the crankshaft operation. These irregularities are usually unable to be determined, without balancing techniques performed on the crankshaft.